This invention relates to a process for removing a gas from a liquid in which it is sparingly soluble, particularly to such a process for deaerating water, when carried out in a rotary device and to a rotary device for use therein.
For many industrial purposes, water is required to be as free as possible of dissolved oxygen. For others, while dissolved oxygen cannot be tolerated, another dissolved gas, such as methane, nitrogen or carbon dioxide, is acceptable.
For example, where water is required to be used as feed to a boiler, all dissolved gases are normally removed therefrom. Oxygen and carbon dioxide present in boiler feed-water tend to give rise to corrosion of the boiler and inert gases such as nitrogen are usually conveniently removed at the same time as the undesirable dissolved gases.
To give another example, where water is used for injection into oil-bearing formations to enhance production of oil from wells bored therein, it is necessary that oxygen by absent from the water, as otherwise bacterial growth in the pores of the formations is often encouraged and the pores may become blocked and oil displacement inhibited. However other gases, for example gaseous hydrocarbons, can be readily tolerated in such injection water.
For all these purposes, oxygen and/or other undesirable dissolved gases have to be removed from water. To achieve this removal, a range of designs of so-called "deaerators" or "degassifiers" have been developed. These devices are static devices in which either the water is exposed to reduced pressure or a displacing gas is passed through the water at atmospheric, reduced or elevated pressure.
The large volumes of water which are treated in these ways entail large vessels and ancillary equipment and represent considerable investment of capital. There is therefore a great incentive to devise ways of reducing the size of the vessels employed. In addition, where the water treatment is to be carried out off-shore, for example in operating boiler feed-water for ships or in deaerating injection water for off-shore wells, any way of reducing the volume and/or weight of the treatment unit is a highly desirable end.
In our published European Patent Specification No 0,089,128A we describe a process for displacing dissolved gas from water in which process the water flows as a thin film through a continuous gas phase, in a high surface area rotor rotating at such a speed as to subject the water to an acceleration, expressed in a direction radial with respect to the axis of rotation, which is greater than the acceleration due to gravity. The thin film is continuously being split, i.e, it tends to form water droplets, such that the mean fluid density is closer to the density of the gas than the density of water.
We have now devised a modified process and device in which for the removal of a gas from unit mass of liquid in unit time a rotor of significantly smaller diameter can be used and at least a fivefold reduction in power consumption may be obtained compared with the rotor and process described in our aforesaid European patent specification.